In accordance with a recent trend using an IP (Internet protocol), an IP-employed system has, of course, been attempted for voice communication systems which representatively use a telephone network. A standardization group called “3GPP” (3rd generation partnership project) for mobile communication performs standardization of IMS (Internet multimedia subsystem) as a basis for a multimedia service which includes a voice service on an IP network. In recent years, a static or fixed IP system known as “NGN” (next generation network) has also been examined.
Current email services have a social problem in which a sender unilaterally sends an email (message) called a spam mail to a receiver, although the receiver does not want to receive the mail. Such a problem may be caused due to a considerable decrease in the communication cost accompanied with generalization of the Internet, or possibility of sending a large quantity of email from a network accessing terminal such as a personal computer which is easily available.
The above IMS, which will be popular in the future, will probably have a similar problem. A technique for protecting users from such a problem which is called SPIT (spam over IP telephony) is called “PUCI” (protection against unsolicited communication for IMS), and examination thereof has been started by 3GPP or the like. The following Patent Document 1 discloses an IMS network system.